Stable Stratification and No Rollover

Under the isobaric (constant pressure) operation of a 1 bar tank, the heat flow into the liquid is removed from the tank continuously by the latent heat of vaporisation of the boil-off vapour, apart from the portion entering the lower layer when there is two level stratification. 

In a zero loss VI tank, the dynamics of the heat flows into the hquid contents are quite different, being isochoric (constant volume) in character. In static tanks with no sloshing, the total heat inflow is absorbed by the stratified upper layer of the liquid. Experimental studies by Chrz and Suma [1], have shown that the stratification is extremely stable whereby the upper layer of warm saturated liquid at T2, 

Experimental studies using LIN in three static VI tanks, of volume 6, 11 and 31 m respectively, demonstrated, that the rate of increase of T2 and P2 with time was just over two times the rate expected for uniform heating of the whole tank contents with no stratification (Fig. 7.1). 

In other words, and more specifically, the mass of the upper liquid layer, which was absorbing all the heat inflows, remained at an average of 44 % (between limits of 38 and 52 %) of the tank volume, during measured and calculated hold times with zero-loss storage, while the lower 56 % liquid layer remained at the initial saturation temperature of Tl. 

While the temperature difference (T2-T1) between the layers is quite large, the isochoric density difference between the two layers is relatively small, with no trapping of heated boundary layer flow in the lower layer. The stratification is therefore convectively stable, with no build-up of thermal overfill in the lower layer, and there appears to be no chance of double-diffusive convection creating a spontaneous mixing and rollover. 

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